Heretofore, a steering wheel or an instrument panel provided integrally with an air bag door for developing an air bag has possessed, on its surface side, a vehicle upholstery member having an air bag door applied with three-dimensional decorations such as embossing. It has been required to provide the position corresponding to the air bag door of such a vehicle upholstery member having the air bag door with a break-scheduled line (which may be also referred to as a tear  line or a split line) as a thin-walled portion for reliably opening the air bag door by the deployment force of the air bag. In order not to impair three-dimensional decorations in a vehicle upholstery member having an air bag door, there has been a demand for an invisible type break-scheduled line that is formed on the back side of the vehicle upholstery member having the air bag door and that cannot be recognized from the front surface side.
Thus, when the invisible type break-scheduled line is formed from the back side of a skin, a heat-cutting jig such as a laser cutter, a high-frequency cutter, an ultrasonic cutter, or a heated blade has been employed. JP 2000-95056 A has disclosed the use of a heated comb-shaped cutting blade as shown in FIG. 21 because the above-mentioned heat-cutting jigs render a vehicle upholstery member having an air bag door susceptible to damage from heat.
JP 06-218811 A has disclosed a method in which, when a groove is formed in a sheet skin made of a thermoplastic plastic, a pressing member 241 having a shape corresponding to that of the groove is ultrasonically pressed against and fused to the sheet skin 211 by a given force, as shown in FIG. 18.
Moreover, a method of producing a vehicle upholstery member having an invisible type air bag door has been disclosed in JP 2000-351335 A, which includes a holding base for processing a thin wall 265 capable of placing an air bag lid-equipped skin 262 turned over therein, and a cutting tool 270 capable of cutting the skin along a guide groove for processing a split line 266 formed in a guide body 267 arranged over the skin that is placed in the holder for processing a thin wall 265, as shown in FIG. 19.
Furthermore, in a production method disclosed in JP 2000-159047 A, when a skin for a vehicle interior-sided member having an air bag door is molded in a vacuum, a sheet for a skin 221 is thermally softened and aspirated into a vacuum mold 223, thereby being shaped, and then a cutting blade 227 is pressed against the position of a split-scheduled portion 222 to be formed in a skin 221 relative to the air bag door of the sheet for a skin while keeping the sheet for a skin aspirated in the vacuum mold 223, to form the split-scheduled portion 222 having a groove shape, as shown in FIG. 20 (a)- 20 (b) .
However, in the formation of a break-scheduled line using the heated comb-shaped cutting blade described in JP 2000-95056 A, a continuous line could not be formed. Therefore, for example if the thickness or the like of a vehicle upholstery member having an air bag door was uneven, it was difficult to construct an air bag door positioned below the vehicle upholstery member having the air bag door to reliably open by the deployment force of an air bag. Moreover, even in the use of the heated comb-shaped cutting blade described in JP 2000-95056 A, etc., there was a problem that, due to a large heat capacity retained in the cutting edge, not a little heat was affected on its surroundings. Consequently, in the case of a skin having a relative thin thickness, the existence of a break-scheduled line could be recognized from the front surface side, resulting in a problem of facilitating reduction in three-dimensional decorative characteristics such as embossing. Furthermore, for measuring the depth of a break-scheduled line with an optical film thickness measurement device, a heated comb-shaped cutting blade having a relatively large area had to be used.
Alternatively, in the formation of a groove (break-scheduled line) described in JP 06-218811 A, there was a problem with the accumulation of a fused thermoplastic around the groove, which causes pebbling. Thus, in the case of a skin having a relatively thin thickness, the existence of a break-scheduled line could be recognized from the front surface side, resulting in a problem of facilitating reduction in three-dimensional decorative characteristics such as embossing. There was also a problem with reduction in adhesiveness between the skin and a foam layer as a lower layer due to irregularities occurring in the back surface. Besides, because a pressing member having a relatively large area had to be used for measuring the depth of the groove with an optical film thickness measurement device, it was more difficult to provide a vehicle upholstery member having an air bag door with high precision and excellent invisibility (non-visibility).
Alternatively, in the formation of a split line (break-scheduled line) described in JP 2000-351335 A, because the skin was bent with its back surface convexed with the holding base for processing a thin wall, large irregularities were formed in the back surface of the skin having the split line when the skin was flattened after the formation of the split line. As a result, it was difficult to form the skin uniformly with a foam layer and the like. Moreover, because the portion where the split line was formed was rendered open when the skin was flattened after forming the split line, there was a problem with deterioration in the invisibility (non-visibility) of the skin from the front surface side.
Further, in the formation of a split-scheduled portion (break-scheduled line) described in JP 2000-159047 A, because the position of the split-scheduled portion to be formed in the skin relative to the air bag door of the sheet for a skin was created while keeping the sheet for a skin aspirated in the vacuum mold, the displacement of the position of the sheet for a skin was hardly caused. However, because of the thermally soften sheet for a skin, it was difficult to accurately control the position of the split-scheduled portion to be formed after the sheet for a skin was allowed to reach room temperature. Moreover, when the skin was flattened after forming the split-scheduled portion, large irregularities were formed in the back surface of the skin having the split-scheduled portion. As a result, it was difficult to form the skin uniformly with a foam layer and the like, in addition to inferior invisibility. Besides, for measuring the depth of the split-scheduled portion described in JP 2000-159047 A with an optical film thickness measurement device, a groove-shaped split-scheduled portion having a relatively large width had to be formed.
Thus, the present invention has been accomplished in light of these problems. An object of the present invention is to provide a vehicle upholstery member having an air bag door, in which an air bag-developing portion in a steering wheel, an instrument panel, or the like in a vehicle cannot be recognized from the front surface side and the depth of a break-scheduled line can be easily and precisely measured with an optical film thickness measurement device; a method of producing the same; and a device for producing the same.